A modular multilevel converter (MMC) is a circuit type in which output terminals of cell converters each composed of a DC capacitor and a switching element such as insulated gate bipolar transistor (IGBT) that can be subjected to ON/OFF control, are connected in series, thereby enabling output of high voltage equal to or greater than the withstand voltage of the switching element, and this circuit type is expected to be applied to a DC power transmission system (HVDC), a static synchronous compensator (STATCOM), and the like.
A cell converter used in a conventional MMC includes two power semiconductor units, and each power semiconductor unit is provided with a power semiconductor chip as one switching element. The switching elements in the two power semiconductor units are connected to each other in a bridge form using an external conductor (for example, Patent Document 1).
As a circuit type in which a plurality of converters are connected in cascade other than the MMC, a high-voltage DC-DC conversion type is known. In this type, a plurality of isolated DC-DC converters are provided in each of which two conversion circuits having a full-bridge configuration are connected via a transformer, and output terminals of the plurality of isolated DC-DC converters are connected in series.
In an isolated DC-DC converter used for a conventional high-voltage DC-DC conversion type, eight switching elements are used (for example, Patent Document 2).